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Dive Strategies and Foraging Effort in the Australasian Gannet Morus Serrator Revealed by Underwater Videography

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Dive Strategies and Foraging Effort in the Australasian Gannet Morus Serrator Revealed by Underwater Videography Vol. 442: 255–261, 2011 MARINE ECOLOGY PROGRESS SERIES Published December 5 doi: 10.3354/meps09458 Mar Ecol Prog Ser Dive strategies and foraging effort in the Australasian gannet Morus serrator revealed by underwater videography Gabriel E. Machovsky Capuska1,2,* , Robin L. Vaughn3, Bernd Würsig3, Gadi Katzir4,5, David Raubenheimer1,2 1Nutritional Ecology Research Group, Institute of Natural Sciences, Massey University, Private Bag 102 904 North Shore MSC, Auckland, New Zealand 2Coastal-Marine Research Group, Institute of Natural Sciences, Massey University, Private Bag 102 904 North Shore MSC, Auckland, New Zealand 3Department of Marine Biology, Texas A&M University at Galveston, 200 Seawolf Parkway, Galveston, Texas 77553, USA 4Department of Marine Biology, University of Haifa, Mount Carmel, Haifa 31905, Israel 5Department of Evolutionary and Environmental Biology, University of Haifa, Mount Carmel, Haifa 31905, Israel ABSTRACT: Gannets are specialist plunge divers that perform short and shallow V-shaped dives and long and deep U-shaped dives in pursuit of pelagic fish and squid. We used underwater video graphy to examine the patterns of behaviour and relative success rates of V- and U-shaped dives in Australasian gannets. A significantly greater proportion of U-shaped dives were associ- ated with successful prey capture than V-shaped dives (95% vs. 43%, respectively). The maxi- mum number of prey captured per dive by the gannets was higher than previously reported, reaching up to 5 fish in a single U-shaped dive. However, V-shaped dives were more efficient in terms of grams of prey captured per time spent underwater in successful dives. In contrast, a pop- ulation-level comparison of the mass of fish captured per total time spent under water (i.e. includ- ing unsuccessful dives) suggested that the 2 dive profiles were equally efficient. We also found that gannets adjusted their dive shape in relation to the depth of their prey rather than prey type, as previously hypothesized. Further studies are needed to understand decisions made by gannets while plunge diving in complex marine environments. KEY WORDS: Gannets · Morus serrator · Dive shape · Prey capture success · Decision making · Multispecies feeding associations Resale or republication not permitted without written consent of the publisher INTRODUCTION 1866). Gannets plunge dive for fish and squid (Nel- son 1978), at times in feeding events called multi- Plunge diving has evolved as a highly specialized species-feeding-associations (MSFA) that involve hunting technique among water bird families includ- other birds (e.g. shearwaters, gulls, terns), predatory ing gannets and boobies (Sulidae), tropicbirds (Phae - pelagic fish (e.g. tuna, sharks) and mammals (e.g. sea tho nidae), pelicans (Pelecanidae), gulls and terns lions, whales, dolphins). Some marine mammals herd (Laridae), and kingfishers (Cerylidae, Alcedinidae). fish towards the surface, where the fish remain To perform a plunge dive, an avian predator must within diving depth for seabirds (Camphuysen & first locate prey from the air and dive at high speeds Webb 1999). However, potential disadvantages to into the water for pursuit and capture (Cunningham plunge divers of these high-density associations *Email: [email protected] © Inter-Research 2011 · www.int-res.com 256 Mar Ecol Prog Ser 442: 255–261, 2011 include competition (Clua & Grosvalet 2001), risk of Plunge diving in Northern gannets has been sug- predation (Heithaus & Frid 2003) and risk of acciden- gested to be a highly accurate foraging technique tal collision (Machovsky Capuska et al. 2011a). (Wanless et al. 2005), but no data on prey capture Until recently it was generally believed that gan- success are available for these species. nets hunt predominately by using ‘steep relatively Here we examine the association between dive vertical plunge diving from a considerable height’ profiles and hunting strategy in the Australasian (Nelson 1978, Garthe et al. 2000). However, by using gannet, using underwater videography. Our study motion data loggers it has been demonstrated that enabled us to test success rate (prey captured per Northern gannet Morus bassanus and Cape gannet dive) and efficiency (grams of fish captured per unit M. capensis use a variety of diving strategies includ- time underwater) of V- and U-shaped dives, and ing surface diving, plunge diving, and pursuit plung- examine the influence of marine mammals on dive ing (Ropert-Coudert et al. 2004, 2009). Northern and patterns. These analyses allowed a better under- Cape gannets display 2 dive types: (1) V-shaped standing of the conditions under which Australasian dives, which are shallow, of short duration, involve gannets adopt V- and U-shaped dive profiles. mostly the underwater momentum of the plunge, and may occasionally include a short phase of active propulsion by using wing flapping to pursue prey MATERIALS AND METHODS that had escaped the initial plunge, and (2) U-shaped dives, which are deeper and longer than the former The study was carried out from 24 August to dive type and always involve the bird shifting from 31 October 2005 and 8 to 12 August 2006 in Admi- the momentum phase to active propulsion by using ralty Bay (40° 57’ S, 173° 55’ E) and Current Basin wing flapping to pursue prey (Garthe et al. 2000, (40° 90’ S, 173° 90’ E), in the Marlborough Sounds, Ropert-Coudert et al. 2004, 2009). New Zealand. This region has been characterized by In Northern gannets it has been suggested that the high primary productivity due to unique local shape of the dive is related to type of prey, with V- oceanographic conditions (Heath 1985). These condi- shaped dives being used to capture larger pelagic tions underlie the presence of a large number of fish such as mackerel Scomber scombrus and herring marine mammals and seabirds (Markowitz et al. Clupea harengus with escape speeds of ca. 1.16 m s−1, 2004, Vaughn et al. 2008). The study was conducted and extended U-shaped dives being used for smaller under Texas A&M Animal Use Protocol 2005-48. and slower pelagic fish such as capelin Mallotus villo- A total of 50 min of underwater video footage sus, with escape speeds of ca. 1.03 m s−1 (Garthe et al. of Australasian gannet dive behaviour from 11 2000). However, the use of remote telemetry data log- stationary dusky dolphin Lagenorhynchus obscurus gers did not allow direct observations of the associa- feeding bouts were recorded. Video recordings were tion between prey type and hunting strategy (Garthe made using a combination of surface- swimming and et al. 2000), and consequently the question of why breath hold dives ranging in depths from 3 to 10 m gannets should employ V-shaped dives in some cir- (Vaughn et al. 2008), using a Sony DCR-HC 1000 cumstances and U-shaped dives in others re mained video camera (focal length 3.6 mm, shutter speed unresolved. In contrast, Elliott et al. (2008) suggested 1/500 s, 30 frames s–1) in an Amphi bico Invader elec- that the shape of a dive may instead be related to the tronic underwater housing (Vaughn et al. 2007). pursuit of prey schools at a specific depth. Footage was analyzed frame by frame using Adobe Australasian gannets Morus serrator are the second Premiere Pro CS4. Individual gannets were followed rarest member of Sulidae (Nelson 2005), and they are from the moment they penetrated the water to the closely related to the Northern and Cape gannets. moment they surfaced. The water surface and the They have been reported to dive up to 20.5 m and water bubble trajectory were used as vertical and hor- 23 m depths in New Zealand and Australian waters, izontal references. Dive depth was de termined using respectively (Green et al. 2009, Ismar 2010), although Adobe Photoshop CS4 Extended version 11.0.2. The they usually dive to about 2 m (Green et al. 2009). mean length of an adult Australasian gannet was Green et al. (2009) reported that Austra la sian gannets used as a size reference as it swam next to a prey ball remain submerged for a maximum of 42 s, although and perpendicular to the video camera. Nelson (1978) they routinely dive for less than 6 s. These gannets reported the mean of Australasian gannet body feed mainly on pilchard Sardinops spp., anchovy En- length as 89 cm, but did not give the error around this graulis spp., saury Scomberesox spp., and jack mack- mean. Since the error is important for assessing the erel Trachurus spp. (Robertson 1992, Bunce 2001). accuracy of our method, we measured the length of Machovsky Capuska et al.: Dive strategies in Australasian gannets 257 20 dead Australasian gannets at autopsy, and ob- RESULTS tained a mean ± SD of 88.9 ± 5.0 cm. Given the close concordance of our measurement and that of Nelson From a total of 85 dives, mean dive duration was (1978), we used 89 cm as our reference value. 7.9 ± 5.4 s, and mean dive depth was 2.5 ± 2.0 m. Dives were categorized as V- or U-shaped (Fig. 1) More than 80% of dives were less than 4 m deep, and in accordance with Garthe et al. (2000). For each 80% of dives lasted less than 13 s (Fig. 2). U-shaped dive, the duration of the underwater momentum dives were longer (t-test, t = 13.758; df = 45; p < phase in which gannets descend through the water 0.0001) and deeper (t-test, t = 17.722; df = 45; p < column without wing propulsion, was compared with 0.0001) than V-shaped dives (Fig. 2). For V-shaped prey pursuit, in which gannets are propelled through dives (n= 39), the mean duration was 3.4 ± 1.8 s the water by actively moving their wings were com- (range = 1.1 ± 8.2 s) and mean depth was 2.9 ± 2.5 m pared.
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